xodr: virtual-junction fidelity — link @elementS/@elementDir dropped, foreign connections deleted, connection children dropped

[JArmandoAnaya]

Problem

Three fidelity holes around virtual junctions and connection internals, all in modeled scopes #453 will not cover:

1. Road-link @elementS/@elementDir are dropped. Rule road.linkage.virtjunc_link_attribute_usage (1.9.0 §10 road linkage) requires them on <link> elements pointing at a virtual junction. read_link (core/src/xodr/reader.cpp:3143) reads only elementType/elementId/contactPoint — no diagnostic — and write_link_element (core/src/xodr/writer.cpp:378) never emits them. A spec-conformant foreign file loses its virtual-junction linkage on save.
2. A foreign virtual junction's <connection>s are deleted. A virtual junction may legally declare connections (§12.7), but the arms-xor-spans policy (reader.cpp:2102) clears them — warned, but the data is destroyed rather than preserved/degraded.
3. <connection> children beyond <laneLink> and the @type attribute are silently dropped (reader.cpp:2087 iterates children("laneLink") only; @id regeneration is benign, @type is not).

Expected

Faithful round-trip for spec-valid input: model or preserve @elementS/@elementDir and connection @type; preserve (not delete) foreign virtual-junction connections with an honest diagnostic; sweep unknown connection children into the preserved tier.

Notes

• Spec: third_party/asam/opendrive-1.9.0/10_roads.md (road linkage rules), 12_junctions.md §12.7.
• Related: fmt-f1: remaining non-preserving parse scopes — header (geoReference), roadMark, laneSection, junction children, unknown attributes #453 (junction attribute/children preservation — this issue is the modeled-scope complement), and the direct-junction corruption filed alongside this one.
• No fuzz seed exercises these paths — add them with the fix.

Filed by the 2026-07-31 roadmap review.

[JArmandoAnaya]

Fixed by #558 (merged as 9e9bce2). @elementS/@elementDir ride RoadLink and are written only when the source carried them, so ordinary road links stay byte-identical. A virtual junction's connections are preserved rather than deleted — held out of connections so nothing derives geometry from them (arms-xor-spans is about generation, which still holds) and re-emitted unchanged. Connection @type and non-<laneLink> children now ride a preserved tier. Corpus seed virtual_junction_links.xodr added.